Most solid surfaces acquire an electrostatic charge when in contact with polar liquids. As a result, a difference in potential is developed across the interface between the negative and positive phases. The charged interface attracts ions of opposition charge (counter-ions) and repels ions of like charge (co-ions) in the liquid. The arrangement of charges that occurs near the interface leads to the development of an electric double layer. When a tangential electric field is applied along the capillary along which the liquid flows, liquids are pumped due to electroosmostic flow. The two widely used methods for the transportation of a single fluid in microfluidics are electroosmostic flow, and pressurized flow.
In microfluidics, the Reynolds number is small and fluid flow is laminar. Laminar fluid diffusion interfaces are created when two or more streams flow in parallel within a single micro-structure. Since the flows are laminar, there is no mixing between them. No mixing may be very useful because only diffusion occurs between the different streams of flow. Therefore, it is able to be used for extraction or separation in biological analysis. Diffusion-based microfluidic devices, such as the T-sensor® and the H-filter® have been developed for commercial use by Micronics, Inc. of Redmond, Wash., USA.
The variable viscosity of biological fluids can be problematic when the two streams of flows have different viscosities. The fluid with higher viscosity will occupy a wider portion of the channel while having a smaller velocity; whereas the fluid with lower viscosity flows at a larger velocity within a narrow portion of the channel. The two fluids will still have the same volumetric flow rate. The unmatched viscosity affects diffusion due to differences in residence time. The average residence time of the more viscous fluid will increase, while that of the less viscous fluid will decrease. To overcome this problem, it has been proposed to measure the viscosity of the fluid, and to add a viscosity-enhancing solute to the less viscous fluid. Another proposal is to control the ratio of the volumetric flow rate of the two fluids. By increasing the flow rate of the less viscous fluid, it is possible to maintain the interface of the two streams at the center of the channel. However, the unmatched average residence time remains unsolved because the less viscous fluid flows even faster, and has even shorter average residence time within the channel.